Automatic transmissions known from practice are increasingly made with gearset systems by virtue of which as many gear ratios as possible can be produced with as small as possible a number of shift elements. For example, in idealized transmission systems ten gear combinations or gear ratios can be produced using five shift elements, when in each case two or three of the shift elements are connected into the force flow of a transmission.
If in each case three of the five shift elements have to be shifted into the force flow to produce a gear ratio, each of the shift elements is involved in the production of six different gears or gear ratios. However, because of the various gear ratio combinations different support factors exist in the area of a shift element. Here, in each case a support factor corresponds to a quotient of the torque actually applied on the shift element involved in producing a gear ratio and a drive torque of a drive machine of a motor vehicle drivetrain.
Depending on the transmission configuration in each case, it is possible for the support factors that correspond to the respective gear ratios to be produced in the area of the shift elements, to differ in some cases by up to a factor of 10.
A resolution of the actuation of a shift element, which is essential for the shift quality of a shift sequence in a transmission mechanism and is known as the sensitivity of the shift element, can in each case be ideally designed only for a defined shift process. To achieve a desired high shift quality for various shift operations in each of which one and the same shift element is involved, with known actuating devices an operating-condition-dependent variation of the resolution between a current specification in the area of an electro-hydraulic transmission control unit, up to a transmission capacity that can be made available in the area of a shift element, can only be achieved with considerable latitude.
Basically, the resolution chain between the electric current specification up to the clutch torque involves a nominal/actual current relationship, an actual current/pilot pressure relationship, a pilot pressure/clutch valve pressure relationship, a clutch valve pressure/clutch pressure relationship, a clutch pressure/piston force relationship, a piston force/piston travel relationship and a piston travel/clutch torque relationship. In this, each individual transfer link is characterized by a particular control sensitivity. For example, to vary the sensitivity it is possible to vary the pressure range of a hydraulic actuating pressure of a shift element, for example by means of holding valves, various valve reinforcements and the like, or to change the piston area of a clutch piston of a shift element.
From DE 102 39 915 B4 a device for controlling a hydraulically actuated shift element of a motor vehicle transmission is known, in which, in the area of the shift element an effective piston area of a clutch piston is reduced by acting upon the clutch piston with hydraulic pressure in the area of an active surface in the engaging direction of the shift element and also in the area of a counter-surface in the disengaging direction. The counter-surface of the clutch piston is smaller than its active surface, in the area of which the clutch piston can be acted upon with hydraulic pressure in the engaging direction of the shift element.
Undesirably, however, the actuation of a shift element is characterized by substantial control and regulation effort since two pressures in two different piston chambers have to be regulated in tune with one another. Furthermore, the area ratio between the active surface and the counter-surface in the area of the piston element of the shift element is fixed.